1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
Maskless lithography refers to lithography that uses an array of individually controllable elements instead of a mask to form a desired radiation exposure pattern on a target. The array of elements is used to pattern a radiation beam, and the patterned beam is projected onto a target surface of a substrate. The projected pattern comprises a plurality of pixels, each pixel typically corresponding to a respective one of the array of controllable elements. Generally, in such techniques, each pixel has a peak intensity that is primarily dependent upon the corresponding respective element, but which is also dependent to some degree on the elements adjacent to the corresponding element.
In a basic form of programmable array, each element may be controllable to adopt one of two states: a “black” state, in which its corresponding pixel on the projective pattern has a minimum intensity; and a “white” state, in which the corresponding pixel has maximum intensity. Thus, the array can be controlled to expose the target portion of the substrate to a desired pattern of corresponding “black” and “white” pixels, each pixel delivering a corresponding dose of radiation during the exposure step.
It is also known to use more sophisticated programmable arrays, in which each element is controllable to adopt a plurality of grey states, in addition to the black and white states. This allows pixels to deliver doses between the “white” maximum and “black” minimum. This finer dose control delivered by each pixel enables finer features to be achieved in the eventual exposure pattern.
In the art, the process of exposing the substrate target surface to a plurality of pixels is sometimes referred to as a printing step, and when an array of elements with black, white, and grey states is used, the process may be described as grey-scale printing.
In certain applications, the radiation dose pattern to which the substrate is to be exposed may be described as comprising “white” regions, defined as regions to which a dose greater than a certain value is to be delivered, and “black” regions, defined as regions to which doses smaller than a certain value are to be delivered. For example, the substrate may have a resist layer, the resist material having a certain threshold activation dose. In such cases, the white regions are those to which the delivered dose is to exceed the activation threshold, and the black regions are to receive less than the activation threshold dose, such that on subsequent development, the black regions are removed, leaving only the pattern of white regions.
When controllable element arrays are used in maskless lithography, there is a possibility that one or more of the elements may be, or may become, defective, and will be unresponsive to control signals, or will not respond in the normal, desired way. For example, a defective element may be an unresponsive element, stuck in a black or grey state. Alternatively, it maybe an element controllable to adopt only a reduced number of its normal states, so that its white state or whitest states, are inaccessible.
When no compensation is made for the defective elements, radiation doses delivered to the target substrate may be smaller than desired. “White” dead pixels may also arise, for example corresponding to elements stuck in a fully white state. “White” dead pixels cannot be corrected when intended to print “black.” Therefore, all “white” dead pixels need to be made “black” before the array is used for lithography, for example, in the case of programmable mirror arrays, by mechanically deforming them to a tilted position by micromanipulation, by removing them, by creating a grating on them, or by coating the mirror black by local deposition of an absorbing material.
To produce a desired exposure pattern on a target substrate, it is known to use a two-pass maskless lithography method. In such a method, each part of the target surface is exposed to a pixel twice, i.e., two exposure steps are used to deliver, in combination, the total required radiation dose to each part of the substrate. Typically, the substrate is moved between the first and second exposure steps, relative to the beam projection system, such that a particular part of the target surface is not exposed to the same pixel twice (i.e., a pixel corresponding to the same controllable element). This has been done to limit the maximum effect that a defective pixel can have. Also, even if no defective elements are present on the array, a two-pass system enables an improved total dose accuracy to be achieved compared with a single exposure method.
To produce a desired exposure pattern of “white” and “black” regions on a target surface of a resist layer of a substrate, a two-pass system is typically arranged such that in each exposure step a fully white pixel delivers a radiation dose just greater than half the resist activation threshold dose. This is achieved by appropriate selection of exposure time (i.e., the time for which the substrate is exposed to a particular pixel in each exposure step) and the intensity of the radiation source.
Previous systems tried to use as short exposure times as possible, which increases pixel printing rate, and hence improves throughput, but this is limited by the switching speeds of the controllable elements (i.e., how fast they can be controlled to switch from one state to another). Previous systems also tried to use a radiation source whose output power is no higher than necessary. This is because, generally, the higher the source power the higher its cost, and the higher the cost of the beam conditioning, transport, and projection systems required to accommodate the beam. In addition, higher beam intensity can lead to an increased rate of degradation of certain components. Thus, in previous systems, the general requirement has been that the projected beam should interact with a fully “white” element to produce a corresponding “white” pixel, which delivers the required dose in the in particular exposure period, i.e., a dose just greater than half the resist threshold dose.
To make best use of available source power, prior art methods have been arranged to print the “white” regions of the target with elements set to their fully “white” states.
Thus, in general, in a typical two-pass method, “white” regions of the target have been printed using “white” pixels of maximum, 100% intensity (i.e., the maximum achievable intensity with the particular radiation source and range of element states), and “grey” and “black” pixels, with intensities down to 0% have also been used to build up the desired dosage pattern.
In such methods, problems occur if an element is completely unresponsive and set in a “black” state, or is otherwise unable to interact with the source beam to make a contribution to its corresponding pixel. In other words, if the element is a “black dead element” and its corresponding pixel is a “black dead pixel.” If, for example, the “black” dead pixel falls on a “white” region of the target surface, then instead of delivering the required dose (e.g., approximately half the threshold dose) in a first pass it will deliver a much reduced dose, even a zero dose. Even though the part of the “white” region upon which the “black” dead pixel falls may be exposed to a full, 100% intensity “white” pixel in the second pass (e.g., corresponding to a non-defective element in the same element array, or a non-defective element in another array), the combined dose it receives may thus fall significantly short of the threshold dose required.
Clearly, such under exposure resulting from defective elements has a detrimental effect on the dosage pattern achieved by the process. It will be appreciated that similar problems also occur if, rather than being completely “black,” a defective element is set in a “grey” state and is unable to deliver, via its corresponding pixel, a sufficiently high dosage in one of the passes.
One previous attempt to compensate for a dead black pixel is illustrated in FIG. 2. Here, a simplified projected pattern 1 of nine pixels is shown, falling on a corner of a white region of a target surface. The boundary of the white region is indicated by broken line 13. Pixels 10, 11 falling on the white region are intended to be fully white (i.e., to print at full intensity), and pixels 12 falling on the black region are intended to be fully black (i.e., zero intensity). However, pixel 11 is a dead black pixel.
To compensate for this, rather than being fully black, non-white pixels 12 neighboring dead black pixel 11 are made grey, such that their contributions to the radiation dosage delivered by pixel 11 combine to compensate at least partially for the dosage lost as a result of the defective element. Thus, the neighboring black pixels in the same exposure step (e.g., write pass) have been used to compensate for a dead black pixel 11 falling on the edge of a white region, and it is also known to use neighboring (i.e., surrounding) black pixels to provide such compensation in a preceding or subsequent pass.
A problem with this compensation method, however, is that by increasing the intensities of pixels 12 from black to grey values, the positions of the feature edges between the black and non-defective white pixels may be undesirably shifted, and once a black region has been given a grey dosage, it is not possible to reverse this. Correcting with neighboring pixels in this way leads not only (and necessarily) to a shift of the edge position, it also makes the edge less steep. The (N)ILS (which is the (normalized) imaging log slope) gets worse. Also, if the dead black pixel falls on a line edge, rather than at a corner, the number of immediately adjacent black pixels for compensation purposes is reduced. Furthermore, if a dead black pixel falls within a white region such that it is surrounded by non-defective white pixels, the above technique can provide no compensation.
Another attempt to solve the problem of underexposure resulting from defective elements/pixels has been to use an additional write pass, which may be referred to as a “clean-up pulse” or exposure. Here, the substrate is moved with respect to the projection system such that no part of the substrate can be exposed to the same defective pixel twice. The clean-up pass is made specifically to deliver targeted radiation doses to selected parts of the substrate which received lower than their desired doses in the preceding exposure step or steps. Although good compensation may be achieved, the problem with this technique is that the need for an additional write pass reduces throughput, or increases costs and complexity if it is achieved by adding further arrays of controllable elements to those normally required for printing.
Although two-pass systems have been described, it will be appreciated that problems of defective elements occur also in single pass methods, and multiple-pass methods using three or more passes to achieve required radiation dosage patterns.
Thus, there remain problems associated with the compensation for effects of defective elements in maskless lithography.
Therefore, what is needed is lithographic methods and apparatus that allow for more efficient and effective compensation for effects of defective elements in maskless lithography.